1. Field of the Invention
This invention generally relates to an image forming member for forming an electrophotographic image thereon and particularly to a photosensitive drum for use in electrophotography. More specifically, the present invention relates to a photosensitive drum including an elastic core around which an outer layer comprised of an elastic supporting layer and a photosensitive layer formed on the supporting layer is provided.
2. Description of the Prior Art
A typical prior art photosensitive drum includes a rigid support and a photosensitive layer formed on the rigid support. Such a photosensitive drum is rotatably mounted in a housing of a copier and driven to rotate in a predetermined direction at constant speed. During a copying operation, an electrostatic latent image is formed on the photosensitive layer of the drum, which is then developed by a developing device to be converted into a visible image. Such a rigid photosensitive drum suffers from the following disadvantages.
That is, in the case where the developing device is structured such that use is made of a developing roller as a developer carrier for transporting a single component developer or magnetic toner as carried thereon along a predetermined path and the toner is charged to a predetermined polarity due, for example, to friction against the developing roller during such transport, whereby the toner thus charged is applied to an electrostatic latent image formed on the drum, it is preferable to form a toner layer on the developing roller as thin as possible, ideally to the thickness which corresponds to the size of a single toner particle, in order to have the toner layer uniformly charged. Because, if the toner layer formed on the developing roller is thicker, it is difficult to have the toner layer charged uniformly in the thickness direction, thereby deteriorating the quality of resultant visible image.
On the other hand, in order to form a thin toner layer on the developing roller, which is then transferred to an electrostatic latent image on the photosensitive drum to obtain a visible image of high quality, it is required to bring the toner on the developing roller into contact with or to a position spaced apart over a small gap from the surface of the photosensitive drum at an opposed region between the developing roller and the photosensitive drum. However, if the photosensitive drum is rigid, it is not easy to bring the extremely thin toner layer on the developing roller into contact with or to a position spaced apart over a small gap from the surface of the photosensitive drum. Because, when the photosensitive drum and the developing roller are brought into pressure contact or arranged extremely closely, especially when the surface of the developing roller is rigid, if slight distortions are present in the surface of the photosensitive drum or developing roller or their outer diameters are slightly inaccurate due to manufacturing tolerances, there is formed a relatively large gap between the drum and the developing roller or the developing roller is pressed against the drum locally with an excessively large force thereby causing damages to the surface of the drum.
It is extremely difficult to manufacture the photosensitive drum or developing roller at high accuracy to the extent that no surface distortions and outer diameter fluctuations are present at all. In order to cope with such inconveniences, use may be made of a photosensitive belt in place of the rigid photosensitive drum, in which the developing roller is pressed against the belt to bring the toner on the developing roller into contact with the surface of the belt uniformly. Using a photosensitive belt requires to provide at least two supporting rollers, and, moreover, it becomes more complicated structurally and expensive. Thus, to use a photosensitive belt instead of the drum does not solve all of the problems.